1
|
Feng L, Zhao T, Xu H, Shi X, Li C, Hsia KJ, Zhang S. Physical forces guide curvature sensing and cell migration mode bifurcating. PNAS NEXUS 2023; 2:pgad237. [PMID: 37680491 PMCID: PMC10482382 DOI: 10.1093/pnasnexus/pgad237] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 02/14/2023] [Accepted: 07/11/2023] [Indexed: 09/09/2023]
Abstract
The ability of cells to sense and adapt to curvy topographical features has been implicated in organ morphogenesis, tissue repair, and tumor metastasis. However, how individual cells or multicellular assemblies sense and differentiate curvatures remains elusive. Here, we reveal a curvature sensing mechanism in which surface tension can selectively activate either actin or integrin flows, leading to bifurcating cell migration modes: focal adhesion formation that enables cell crawling at convex front edges and actin cable assembly that pulls cells forward at concave front edges. The molecular flows and curved front morphogenesis are sustained by coordinated cellular tension generation and transmission. We track the molecular flows and mechanical force transduction pathways by a phase-field model, which predicts that multicellular curvature sensing is more efficient than individual cells, suggesting collective intelligence of cells. The unique ability of cells in curvature sensing and migration mode bifurcating may offer insights into emergent collective patterns and functions of living active systems at different length scales.
Collapse
Affiliation(s)
- Luyi Feng
- Department of Engineering Science and Mechanics, Pennsylvania State University, University Park, PA 16802, USA
| | - Tiankai Zhao
- Department of Engineering Science and Mechanics, Pennsylvania State University, University Park, PA 16802, USA
| | - Hongmei Xu
- School of Mechanical and Aerospace Engineering, Nanyang Technological University, Singapore 639798, Singapore
| | - Xuechen Shi
- Department of Biomedical Engineering, Pennsylvania State University, University Park, PA 16802, USA
| | - Changhao Li
- Department of Engineering Science and Mechanics, Pennsylvania State University, University Park, PA 16802, USA
| | - K Jimmy Hsia
- School of Mechanical and Aerospace Engineering, Nanyang Technological University, Singapore 639798, Singapore
- School of Chemistry, Chemical Engineering and Biotechnology, Nanyang Technological University, Singapore 639798, Singapore
| | - Sulin Zhang
- Department of Engineering Science and Mechanics, Pennsylvania State University, University Park, PA 16802, USA
- Department of Biomedical Engineering, Pennsylvania State University, University Park, PA 16802, USA
- Department of Materials Science and Engineering, Pennsylvania State University, University Park, PA 16802, USA
| |
Collapse
|
2
|
Xu H, Huo Y, Zhou Q, Wang LA, Cai P, Doss B, Huang C, Hsia KJ. Geometry-mediated bridging drives nonadhesive stripe wound healing. Proc Natl Acad Sci U S A 2023; 120:e2221040120. [PMID: 37098071 PMCID: PMC10161107 DOI: 10.1073/pnas.2221040120] [Citation(s) in RCA: 3] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/14/2022] [Accepted: 03/14/2023] [Indexed: 04/26/2023] Open
Abstract
Wound healing through reepithelialization of gaps is of profound importance to the medical community. One critical mechanism identified by researchers for closing non-cell-adhesive gaps is the accumulation of actin cables around concave edges and the resulting purse-string constriction. However, the studies to date have not separated the gap-edge curvature effect from the gap size effect. Here, we fabricate micropatterned hydrogel substrates with long, straight, and wavy non-cell-adhesive stripes of different gap widths to investigate the stripe edge curvature and stripe width effects on the reepithelialization of Madin-Darby canine kidney (MDCK) cells. Our results show that MDCK cell reepithelization is closely regulated by the gap geometry and may occur through different pathways. In addition to purse-string contraction, we identify gap bridging either via cell protrusion or by lamellipodium extension as critical cellular and molecular mechanisms for wavy gap closure. Cell migration in the direction perpendicular to wound front, sufficiently small gap size to allow bridging, and sufficiently high negative curvature at cell bridges for actin cable constriction are necessary/sufficient conditions for gap closure. Our experiments demonstrate that straight stripes rarely induce cell migration perpendicular to wound front, but wavy stripes do; cell protrusion and lamellipodia extension can help establish bridges over gaps of about five times the cell size, but not significantly beyond. Such discoveries deepen our understanding of mechanobiology of cell responses to curvature and help guide development of biophysical strategies for tissue repair, plastic surgery, and better wound management.
Collapse
Affiliation(s)
- Hongmei Xu
- School of Mechanical and Aerospace Engineering, Nanyang Technological University, Singapore639798, Singapore
| | - Yucheng Huo
- School of Mechanical and Aerospace Engineering, Nanyang Technological University, Singapore639798, Singapore
| | - Quan Zhou
- School of Mechanical and Aerospace Engineering, Nanyang Technological University, Singapore639798, Singapore
| | - Lianghao Abraham Wang
- School of Mechanical and Aerospace Engineering, Nanyang Technological University, Singapore639798, Singapore
| | - Pingqiang Cai
- Jiangsu Key Laboratory of Molecular Medicine, Medical School, Nanjing University, Nanjing211166, PR China
| | - Bryant Doss
- School of Mechanical and Aerospace Engineering, Nanyang Technological University, Singapore639798, Singapore
| | - Changjin Huang
- School of Mechanical and Aerospace Engineering, Nanyang Technological University, Singapore639798, Singapore
| | - K. Jimmy Hsia
- School of Mechanical and Aerospace Engineering, Nanyang Technological University, Singapore639798, Singapore
- School of Chemistry, Chemical Engineering and Biotechnology, Nanyang Technological University, Singapore639798, Singapore
| |
Collapse
|
3
|
Cell alignment modulated by surface nano-topography - Roles of cell-matrix and cell-cell interactions. Acta Biomater 2022; 142:149-159. [PMID: 35124266 DOI: 10.1016/j.actbio.2022.01.057] [Citation(s) in RCA: 12] [Impact Index Per Article: 6.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/04/2021] [Revised: 01/19/2022] [Accepted: 01/31/2022] [Indexed: 12/27/2022]
Abstract
The propensity of cells to align in particular directions is relevant to a number of areas, including tissue engineering and biohybrid robotics. Cell alignment is modulated through various extracellular conditions including surface topographies, mechanical cues from cell-matrix interactions, and cell-cell interactions. Understanding of these conditions provides guidance for desirable cellular structure constructions. In this study, we examine the roles of surface topographies and cell-cell interactions in inducing cell alignment. We employed wavy surface topographies at the nanometer scale as a model extracellular environment for cell culture. The results show that, within a certain range of wavelengths and amplitudes of the surface topographies, cell alignment is dependent on cell confluency. This dependence on both topology and confluency suggests interplay between cell-cell and cell-matrix interactions in inducing cell alignment. Images of sparsely distributed and confluent cells also demonstrated clear differences in the structures of their focal adhesion complexes. To understand this effect, we introduced anti-N-cadherin to cell culture to inhibit cell-cell interactions. The results show that, when anti-N-cadherin was applied, cells on wavy surfaces required greater confluency to achieve the same alignment compared to that in the absence of anti-N-cadherin. The understanding of the cell alignment mechanisms will be important in numerous potential applications such as scaffold design, tissue repair, and development of biohybrid robotic systems. STATEMENT OF SIGNIFICANCE: Cell alignment plays a critical role in numerous biological functions. Advances in tissue engineering utilizes cell alignment to restore, maintain, or even replace different types of biological tissues. The clinical impact that tissue engineering has made is facilitated by advancements in the understanding of interactions between scaffolds, biological factors, and cells. This work further elucidates the role of cell-cell interactions in promoting the organization of biological tissues.
Collapse
|
4
|
He C, Liu M, Jiang D, Wu J, Qin C, Liang T, Wu P, Han C, Huang L, Hsia KJ, Wang P. Fabricating Tissues In Situ with the Controlled Cellular Alignments. Adv Healthc Mater 2022; 11:e2100934. [PMID: 34648692 DOI: 10.1002/adhm.202100934] [Citation(s) in RCA: 2] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/21/2021] [Revised: 09/15/2021] [Indexed: 11/07/2022]
Abstract
Tissue engineering techniques have enabled to replicate the geometrical architecture of native tissues but usually fail to reproduce their exact cellular arrangements during the fabricating process, while it is critical for manufacturing physiologically relevant tissues. To address this problem, a "sewing-like" method of controlling cellular alignment during the fabricating process is reported here. By integrating the stretching step into the fabricating process, a static mechanical environment is created which, in turn, regulates the subsequent cellular alignment, elongation, and differentiation in the generated tissues. With this method, patterned cellular constructs can be fabricated with controlled cellular alignment. Moreover, this method shows a potent capability to fabricate physiologically relevant skeletal muscle constructs in vitro by mechanically inducing myoblast fusion and maturation. As a potential clinical application, aligned myofibers are directly fabricated onto injured muscles in vivo, which repair the damaged tissues effectively. This study shows that the "sewing-like" method can produce engineered tissues with precise control of cellular arrangements and more clinically viable functionalities.
Collapse
Affiliation(s)
- Chuanjiang He
- Biosensor National Special Laboratory Key Laboratory for Biomedical Engineering Ministry of Education Department of Biomedical Engineering Zhejiang University Hangzhou 310027 China
- State Key Laboratory of Transducer Technology Chinese Academy of Sciences Shanghai 200050 China
| | - Mengxue Liu
- Biosensor National Special Laboratory Key Laboratory for Biomedical Engineering Ministry of Education Department of Biomedical Engineering Zhejiang University Hangzhou 310027 China
| | - Deming Jiang
- Biosensor National Special Laboratory Key Laboratory for Biomedical Engineering Ministry of Education Department of Biomedical Engineering Zhejiang University Hangzhou 310027 China
| | - Jianguo Wu
- Biosensor National Special Laboratory Key Laboratory for Biomedical Engineering Ministry of Education Department of Biomedical Engineering Zhejiang University Hangzhou 310027 China
| | - Chunlian Qin
- Biosensor National Special Laboratory Key Laboratory for Biomedical Engineering Ministry of Education Department of Biomedical Engineering Zhejiang University Hangzhou 310027 China
| | - Tao Liang
- Biosensor National Special Laboratory Key Laboratory for Biomedical Engineering Ministry of Education Department of Biomedical Engineering Zhejiang University Hangzhou 310027 China
| | - Pan Wu
- Department of Burns and Wound Care Center The Second Affiliated Hospital of Zhejiang University School of Medicine Zhejiang University Hangzhou 310009 China
| | - Chunmao Han
- Department of Burns and Wound Care Center The Second Affiliated Hospital of Zhejiang University School of Medicine Zhejiang University Hangzhou 310009 China
| | - Liquan Huang
- College of Life Sciences Zhejiang University Hangzhou 310058 China
- Monell Chemical Senses Center Philadelphia PA 19104 USA
| | - K. Jimmy Hsia
- Schools of Chemical and Biomedical Engineering and Mechanical and Aerospace Engineering Nanyang Technological University Singapore 639798 Singapore
| | - Ping Wang
- Biosensor National Special Laboratory Key Laboratory for Biomedical Engineering Ministry of Education Department of Biomedical Engineering Zhejiang University Hangzhou 310027 China
- State Key Laboratory of Transducer Technology Chinese Academy of Sciences Shanghai 200050 China
| |
Collapse
|
5
|
Connon CJ, Gouveia RM. Milliscale Substrate Curvature Promotes Myoblast Self-Organization and Differentiation. Adv Biol (Weinh) 2021; 5:e2000280. [PMID: 33852180 DOI: 10.1002/adbi.202000280] [Citation(s) in RCA: 3] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/14/2020] [Revised: 01/23/2021] [Indexed: 11/06/2022]
Abstract
Biological tissues comprise complex structural environments known to influence cell behavior via multiple interdependent sensing and transduction mechanisms. Yet, and despite the predominantly nonplanar geometry of these environments, the impact of tissue-size (milliscale) curvature on cell behavior is largely overlooked or underestimated. This study explores how concave, hemicylinder-shaped surfaces 3-50 mm in diameter affect the migration, proliferation, orientation, and differentiation of C2C12 myoblasts. Notably, these milliscale cues significantly affect cell responses compared with planar substrates, with myoblasts grown on surfaces 7.5-15 mm in diameter showing prevalent migration and alignment parallel to the curvature axis. Moreover, surfaces within this curvature range promote myoblast differentiation and the formation of denser, more compact tissues comprising highly oriented multinucleated myotubes. Based on the similarity of effects, it is further proposed that myoblast susceptibility to substrate curvature depends on mechanotransduction signaling. This model thus supports the notion that cellular responses to substrate curvature and compliance share the same molecular pathways and that control of cell behavior can be achieved via modulation of either individual parameter or in combination. This correlation is relevant for elucidating how muscle tissue forms and heals, as well as for designing better biomaterials and more appropriate cell-surface interfaces.
Collapse
Affiliation(s)
- Che J Connon
- Tissue Engineering Lab Biosciences Institute, Newcastle University, International Centre for Life, Newcastle upon Tyne, NE1 3BZ, UK
| | - Ricardo M Gouveia
- Biosciences Institute, Newcastle University, International Centre for Life, Newcastle upon Tyne, NE1 3BZ, UK
| |
Collapse
|
6
|
Biofabrication of aligned structures that guide cell orientation and applications in tissue engineering. Biodes Manuf 2021. [DOI: 10.1007/s42242-020-00104-5] [Citation(s) in RCA: 15] [Impact Index Per Article: 5.0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/13/2022]
|
7
|
Fabrication of microgrooved scaffolds using near-field electrospinning-assisted lithography (NFEAL). J IND ENG CHEM 2019. [DOI: 10.1016/j.jiec.2019.08.025] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/23/2022]
|
8
|
Almonacid Suarez AM, Zhou Q, van Rijn P, Harmsen MC. Directional topography gradients drive optimum alignment and differentiation of human myoblasts. J Tissue Eng Regen Med 2019; 13:2234-2245. [PMID: 31677226 PMCID: PMC6973069 DOI: 10.1002/term.2976] [Citation(s) in RCA: 22] [Impact Index Per Article: 4.4] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/10/2019] [Revised: 09/06/2019] [Accepted: 09/26/2019] [Indexed: 12/20/2022]
Abstract
Tissue engineering of skeletal muscle aims to replicate the parallel alignment of myotubes on the native tissue. Directional topography gradients allow the study of the influence of topography on cellular orientation, proliferation, and differentiation, resulting in yield cues and clues to develop a proper in vitro environment for muscle tissue engineering. In this study, we used a polydimethylsiloxane-based substrate containing an aligned topography gradient with sinusoidal features ranging from wavelength (λ) = 1,520 nm and amplitude (A) =176 nm to λ = 9,934 nm and A = 2,168 nm. With this topography gradient, we evaluated the effect of topography on human myoblasts distribution, dominant orientation, cell area, nuclei coverage, cell area per number of nuclei, and nuclei area of myotubes. We showed that human myoblasts aligned and differentiated irrespective of the topography section. In addition, aligned human myotubes showed functionality and maturity by contracting spontaneously and nuclei peripheral organization resembling natural myotubes.
Collapse
Affiliation(s)
- Ana Maria Almonacid Suarez
- Department of Pathology and Medical Biology, University Medical Center GroningenUniversity of GroningenGroningenThe Netherlands
| | - Qihui Zhou
- Department of Biomedical Engineering, W.J. Kolff Institute for Biomedical Engineering and Materials Science, University Medical Center GroningenUniversity of GroningenGroningenThe Netherlands
| | - Patrick van Rijn
- Department of Biomedical Engineering, W.J. Kolff Institute for Biomedical Engineering and Materials Science, University Medical Center GroningenUniversity of GroningenGroningenThe Netherlands
- Zernike Institute for Advanced MaterialsUniversity of GroningenGroningenThe Netherlands
| | - Martin C. Harmsen
- Department of Pathology and Medical Biology, University Medical Center GroningenUniversity of GroningenGroningenThe Netherlands
| |
Collapse
|
9
|
Yu JZ, Korkmaz E, Berg MI, LeDuc PR, Ozdoganlar OB. Biomimetic scaffolds with three-dimensional undulated microtopographies. Biomaterials 2017; 128:109-120. [DOI: 10.1016/j.biomaterials.2017.02.014] [Citation(s) in RCA: 21] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/18/2016] [Revised: 01/18/2017] [Accepted: 02/10/2017] [Indexed: 12/20/2022]
|
10
|
Jana S, Lan Levengood SK, Zhang M. Anisotropic Materials for Skeletal-Muscle-Tissue Engineering. ADVANCED MATERIALS (DEERFIELD BEACH, FLA.) 2016; 28:10588-10612. [PMID: 27865007 PMCID: PMC5253134 DOI: 10.1002/adma.201600240] [Citation(s) in RCA: 164] [Impact Index Per Article: 20.5] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 01/14/2016] [Revised: 06/27/2016] [Indexed: 05/19/2023]
Abstract
Repair of damaged skeletal-muscle tissue is limited by the regenerative capacity of the native tissue. Current clinical approaches are not optimal for the treatment of large volumetric skeletal-muscle loss. As an alternative, tissue engineering represents a promising approach for the functional restoration of damaged muscle tissue. A typical tissue-engineering process involves the design and fabrication of a scaffold that closely mimics the native skeletal-muscle extracellular matrix (ECM), allowing organization of cells into a physiologically relevant 3D architecture. In particular, anisotropic materials that mimic the morphology of the native skeletal-muscle ECM, can be fabricated using various biocompatible materials to guide cell alignment, elongation, proliferation, and differentiation into myotubes. Here, an overview of fundamental concepts associated with muscle-tissue engineering and the current status of muscle-tissue-engineering approaches is provided. Recent advances in the development of anisotropic scaffolds with micro- or nanoscale features are reviewed, and how scaffold topographical, mechanical, and biochemical cues correlate to observed cellular function and phenotype development is examined. Finally, some recent developments in both the design and utility of anisotropic materials in skeletal-muscle-tissue engineering are highlighted, along with their potential impact on future research and clinical applications.
Collapse
Affiliation(s)
- Soumen Jana
- Department of Materials Science & Engineering, University of Washington, Seattle, Washington 98195, USA
| | - Sheeny K. Lan Levengood
- Department of Materials Science & Engineering, University of Washington, Seattle, Washington 98195, USA
| | - Miqin Zhang
- Department of Materials Science & Engineering, University of Washington, Seattle, Washington 98195, USA
| |
Collapse
|
11
|
Joaquin D, Grigola M, Kwon G, Blasius C, Han Y, Perlitz D, Jiang J, Ziegler Y, Nardulli A, Hsia KJ. Cell migration and organization in three-dimensional in vitro culture driven by stiffness gradient. Biotechnol Bioeng 2016; 113:2496-506. [PMID: 27183296 DOI: 10.1002/bit.26010] [Citation(s) in RCA: 26] [Impact Index Per Article: 3.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/21/2016] [Revised: 05/11/2016] [Accepted: 05/12/2016] [Indexed: 12/31/2022]
Abstract
Durotaxis, a phenomenon that cells move according to changes in stiffness of the extra cellular matrix, has emerged as a crucial parameter controlling cell migration behavior. The current study provides a simple method to generate three-dimensional continuous stiffness variations without changing other physical characteristics of the extra cellular environment. Using Finite Element simulations, the stiffness and the stiffness gradient variations are evaluated quantitatively, leading to an analysis of the dependence of cell migration behavior on the substrate stiffness parameters. We tested various cell lines on several 3-D environments. The durotaxis results show that the cell migration velocity does not have any consistency with the stiffness of the substrate, rather it is more related to the stiffness gradient of the substrate. This finding suggests a new mechanism underlying the durotaxis phenomenon, highlighting the importance of the substrate stiffness gradient, rather than the stiffness itself. Biotechnol. Bioeng. 2016;113: 2496-2506. © 2016 Wiley Periodicals, Inc.
Collapse
Affiliation(s)
- Danielle Joaquin
- Department of Mechanical Science and Engineering, University of Illinois at Urbana-Champaign, Urbana, Illinois
| | - Michael Grigola
- Department of Mechanical Science and Engineering, University of Illinois at Urbana-Champaign, Urbana, Illinois
| | - Gubeum Kwon
- Department of Mechanical Engineering, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213
| | - Christopher Blasius
- Department of Biomedical Engineering, University of Missouri-Columbia, Columbia, Missouri
| | - Yutao Han
- Department of Mechanical Science and Engineering, University of Illinois at Urbana-Champaign, Urbana, Illinois
| | - Daniel Perlitz
- Department of Mechanical Science and Engineering, University of Illinois at Urbana-Champaign, Urbana, Illinois
| | - Jing Jiang
- Department of Electrical and Computer Engineering, University of Illinois at Urbana-Champaign, Urbana, Illinois
| | - Yvonne Ziegler
- Department of Molecular and Integrative Physiology, University of Illinois at Urbana-Champaign, Urbana, Illinois
| | - Ann Nardulli
- Department of Molecular and Integrative Physiology, University of Illinois at Urbana-Champaign, Urbana, Illinois
| | - K Jimmy Hsia
- Department of Mechanical Engineering, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213. .,Department of Biomedical Engineering, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213.
| |
Collapse
|
12
|
Chitooligomer-Immobilized Biointerfaces with Micropatterned Geometries for Unidirectional Alignment of Myoblast Cells. Biomolecules 2016; 6:12. [PMID: 26784249 PMCID: PMC4808806 DOI: 10.3390/biom6010012] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/09/2015] [Revised: 01/08/2016] [Accepted: 01/12/2016] [Indexed: 11/16/2022] Open
Abstract
Skeletal muscle possesses a robust capacity to regenerate functional architectures with a unidirectional orientation. In this study, we successfully arranged skeletal myoblast (C2C12) cells along micropatterned gold strips on which chitohexaose was deposited via a vectorial chain immobilization approach. Hexa-N-acetyl-d-glucosamine (GlcNAc6) was site-selectively modified at its reducing end with thiosemicarbazide, then immobilized on a gold substrate in striped micropatterns via S–Au chemisorption. Gold micropatterns ranged from 100 to 1000 µm in width. Effects of patterning geometries on C2C12 cell alignment, morphology, and gene expression were investigated. Unidirectional alignment of C2C12 cells having GlcNAc6 receptors was clearly observed along the micropatterns. Decreasing striped pattern width increased cell attachment and proliferation, suggesting that the fixed GlcNAc6 and micropatterns impacted cell function. Possibly, interactions between nonreducing end groups of fixed GlcNAc6 and cell surface receptors initiated cellular alignment. Our technique for mimicking native tissue organization should advance applications in tissue engineering.
Collapse
|